1. Field of Invention
The present invention relates to methods and apparatus for implementing a satellite-based emergency broadcast system and, more particularly, to methods and apparatus associated with an emergency broadcast system that is integrated with infrastructure and communications that support a satellite-based global positioning system.
2. Description of the Related Art
Global Positioning System (GPS) satellite navigation as exemplified by NAVSTAR/GPS, is an accurate, three-dimensional navigation system that has become one of the most important technologies of the era, impacting a myriad of users from aircraft and ships, to farmers and hikers. The GPS comprises a constellation of twenty-eight active satellites that orbit the earth twice each day. The orbits of the GPS satellites are maintained in a virtually circular manner at approximately 10,898 nautical miles above the earth, the GPS satellites orbit the earth in six overlapping orbital planes based on the equatorial plane of the earth. Although the number of satellites and number of planes in the GPS constellation may change in the future, as the design of the GPS system evolves, GPS satellite orbits are chosen so that the GPS system can provide information to users regardless of the time that the user needs information and regardless of the user's position on the earth's surface. This information contains a navigation message that includes satellite position and satellite clock drift information.
In order for the system to operate properly, the orbits of the GPS satellites are maintained by the GPS Control Segment which uses a global network of ground-based tracking stations and uplink antennas. The ground-based tracking stations each use a GPS L-band receiver to monitor the orbits of the GPS satellites. Each GPS satellite continuously broadcasts pseudo-random codes at L-band frequencies, L1 at 1575.42 MHz and L2 at 1227.6 MHz. One of these signals is referred to as a coarse acquisition (C/A) code, which is a signal that can be received by civilian-type GPS receivers. The other signal is referred to as a precision (P) code, which is a signal that can be received only by military-type GPS receivers. The ground stations on the earth receive these L-band transmissions from the satellites. These transmissions are analyzed by the GPS Control Segment which continuously estimates the precise orbital and clock drift parameters for each of the satellites in the constellation. Updated estimates for these parameters are then uplinked to the satellites by the Control Segment using a global network of uplink ground antennas. Each satellite then updates the orbital and clock data it transmits to the GPS users.
A major benefit of the GPS is that the number of users is unlimited, because the signals transmitted by the satellites are passively acquired. Thus, broad civilian and commercial applications are possible. For example, GPS navigation is commonly applied in terrestrial (earth) based applications. In such applications, a GPS receiver is located in mobile units, such as ground vehicles, to enable the vehicle operators to precisely locate their respective global positions. GPS navigation has also been proven to be of value for aircraft and spacecraft use, with such “non-terrestrial” mobile units employing a GPS receiver for precisely locating the unit's global position.
The user's GPS receiver operates by engaging in a radio-ranging calculation which involves acquiring the encoded signals transmitted by each GPS satellite and making pseudorange measurements. These measurements are processed in real time to provide the best estimate of the user's position (latitude, longitude, and altitude), velocity, and system time. The user's receiver maintains a time reference that is used to generate a replica of the codes transmitted by the satellite. The amount of time that the receiver must apply to correlate the replicated code with the satellite clock referenced code received from the satellite provides a measure of the signal propagation time between the satellite and the receiver. This time propagation or “pseudorange” measurement is a measure of the time synchronization error between the satellite and receiver clocks, and thus allows time to be precisely synchronized for position calculation purposes. The user's receiver then, employs a multi-dimensional equivalent of triangulation on the data received from the GPS satellites to compute the user's position. In order to use this “trilateration” technique, four of the orbiting GPS satellites generally must be visible (i.e., within line of sight) to the user at any one time, and the position of these four satellites relative to the earth must be known.
In light of the recent increase in terrorist activities, in addition to the inevitability that other disaster situations, whether man-made or natural, will occur in the future, it is of the utmost importance to establish multiple mechanisms by which emergency response organizations can disseminate information quickly and to as broad an audience as possible within an area affected by an emergency. This capability could mean the difference between life and death for many people. Such systems already exist to some extent. The United States, for example, has an Emergency Broadcast System EBS that is used to broadcast messages via public and commercially-owned radio and television transmitters in the case of an emergency. Other countries have similar systems.
One shortcoming associated with existing emergency broadcast systems is that persons who are not within close proximity to a television or radio in active use when an alert is issued may not receive the alert. A further shortcoming is that these systems are effective only in geographic areas covered by commercial television and radio broadcast infrastructures, which could be damaged or disabled in emergency situations. Yet another shortcoming is the lack of an ability to geographically control emergency message dissemination. For example, certain evacuation instructions may need to be given to people located in one geographic area, while different evacuation instructions may need to be given to people located in another geographic area.
The number of new and useful applications for GPS has steadily increased in recent years. Such uses include position location of cellular phones for emergency response (the E911 standard), car navigation systems, GPS receivers as “worn” devices such as GPS enabled wrist watches, etc. All these devices contain various types of imbedded GPS receivers. The fact that such devices are more likely than a radio or TV to be turned on, operating, and constantly within reach of people, make such devices ideal candidates for receiving emergency notification data and alerting the owner.
Accordingly, there is a need for an emergency message distribution system capable of delivering emergency messages and information on a national and global basis, yet that is also capable of delivering highly specific emergency messages to relatively small geographic target locations. The emergency message development, coordination and broadcast approach would preferably be compatible with and leverage off existing deployed infrastructure and capable of deployment without extensive capital investment or significant increases in long-term lifecycle costs. The associated reception technology should be capable of cost effective integration into a wide variety of existing hand-held and portable consumer products, thereby significantly extending the emergency message reception community and the likelihood of rapid dissemination of emergency information.